Power over Ethernet (PoE): All You Need to Know

Date September 18, 2014 Author Comments Leave your thoughts

Power Over Ethernet Networking

Power over Ethernet (PoE) is a term that is banded around a lot these days and is something that is taken for granted by many in IT (and maybe some at home) however there are some pitfalls to be aware of, especially with the latest generation of power hungry equipment.

Power over Ethernet: In the Beginning…

In the old days – well the 90’s – Power over Ethernet was introduced to overcome the challenge of businesses deploying early generation Wireless networks and the requirement to place Access Points in strange places that generally didn’t have power points (ceilings, cupboards, etc.).

Due to the forward planning of the types who designed Twisted Pair cabling we have cabling going from our network cabinets to our Access Points that have 8 strands of wire and hey we only need 4, so lets use the other 4 for…power, yeah, just saved us the cost of employing an electrician as well as the data company.

In those early days Power over Ethernet hadn’t been standardised by the IEEE (those policy guys who define everything that comes onto their radar) so mixing and matching devices that supported early PoE and PoE providing devices was often hit-and-miss (more miss than hit, generally). This meant that powering a bit of kit with a competitor’s PoE injector may or may not work.

When the IEEE got involved they standardised PoE and set the first standard which became known as 802.3af.

The First Power over Ethernet Standard: 802.3af

802.3af is what most people understand as PoE and it has been the standard since 2003.

The major benefit of standardisation (usually) means that competitive vendor equipment will work with other vendor’s equipment without a problem, meaning that a 802.3af PoE Injector would power any PoE Access Point/CCTV Camera, etc.

The standard defined a number of things, but crucially the Wattage on the port – how much power was to be provided to the end device – which was set at 15.4W. This worked really well for a long time with most devices not requiring any more power to work correctly. But, as is the way, newer devices came to market with new functionality, better features and more performance.

In the Wireless LAN world came 802.11ac, which uses more antennas, more radio transmitter/receivers and sometimes comes with the addition of specific security chips, meaning that the 802.3af PoE standard could no longer provide enough power.

So What Next?…802.3at

In 2009 a new standard was ratified by the IEEE called 802.3at, or PoE+ – note the additional plus symbol after the word PoE.

This new Power over Ethernet standard provided 25.5W output, which was again standardised so that vendor equipment could be mixed and matched. PoE+ was also implemented to be backwardly compatible which enabled the implementation of devices only requiring the lesser power of the previous PoE standard.

In our Plug-and-Play world (remember that term?) the same also happens in reverse. If an Access Point that requires PoE+ to work properly is plugged into a PoE power supply the PoE+ Access Point will continue to work but with reduced functionality – with maybe one radio shut down – thus allowing the deployment of newer technologies, such as 802.11ac Wireless LAN, with PoE and then  migrating to full functionality with PoE+ later.

A Final Word on PoE/PoE+ Devices: There Are Three Options

  • PoE/PoE+ Network Switches

A Layer 2 or 3 network switch providing switching capabilities with all or partial port support for PoE or PoE+ – the best option if you can afford it as everything is integrated into one device.
However, be careful to consider the whole power budget as some switch manufacturers are a little dis-honest, claiming their switch offers PoE+ which individual ports may be able to, but once you add up a number of PoE+ ports you would exceed the total power available.
To overcome this, divide the total power by the number of PoE and PoE+ devices to ensure the switch is powerful enough.

  • PoE/PoE+ MidSpans

A device – normally rack mountable – which can provide multiple PoE/PoE+ connections, and looks similar to a network switch, but only provides power without the switching capabilities. This is a good option to complement an existing install of PoE/PoE+ switches.

  • PoE/PoE+ Power Injectors

A one port device that can look like a power supply, sometimes known as a 1 port MidSpan. These can be cheap and ideal for small single deployments, but hard to manage and not as reliable as a full MidSpan or switch. There are two network ports, one to provide power and data to the Access Point and the second to provide connectivity back to the network switch.

A Word of Warning

When purchasing your new Wireless LAN system, quiz your provider about the power requirements. One would hope they are already having this discussion with you but you don’t want to get caught out by not having the facts to hand. Engaging with a partner that understands your business requirements and the many technical challenges is – we believe – key to the successful deployment of network infrastructures, and at Ensign we like to believe we act as a trusted advisor to our customers.

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Author Jim Lucking

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